An optical transmission system typically comprises a transmitter (Tx) and a receiver (Rx) interconnected by an optical fiber link which may be composed of one of more fiber spans. Adjacent spans are commonly interconnected by an optical amplifier (such as an Erbium Doped Fiber Amplifier, EDFA). Other optical equipment, such as optical add-drop multiplexers (OADMs), for example, may also be installed between adjacent spans of the optical fiber link.
It is often desirable to know the magnitude of nonlinear optical impairments in an optical transmission system. These impairments, including cross-phase modulation (XPM), self-phase modulation (SPM), cross-polarization modulation (XPolM), and four wave mixing (FWM), are needed to estimate various link budget parameters, including the required optical signal-to-noise ratio (ROSNR) to achieve a specified bit-error-ratio at the Rx, and the signal power at the input to each optical fiber span that maximizes the received net system margin. The calculation of these parameters is a normal part of validating a planned link between a given Tx/Rx pair.
Traditional methods of nonlinear impairment calculation often employ a split-step Fourier (SSF) solution of the nonlinear Schrodinger equation (NLSE). SSF propagation simulations require the calculation of optical nonlinearities in the time domain (i.e. using a waveform of optical field versus time) in tandem with linear propagation through dispersive optical fiber in the frequency domain (i.e. a spectrum of the optical field versus optical frequency). Conversion from time-domain to frequency-domain is commonly performed using the fast Fourier-transform (FFT) algorithm. In order to ensure the accuracy of the propagated optical field, the nonlinear optical phase and chromatic dispersion must be calculated sequentially and repeatedly after short propagation distances. This may require hundreds of calculation steps for each span of fiber, with each step often requiring two FFT calculations.
There are other known methods for calculating nonlinear impairments, but a common theme is the repeated use of FFTs.
In an optical network being designed via an optical planning tool (OPT), there may be tens of thousands of optical channel (OCh) trails that require validation. Each trail represents an optical channel link between a given Tx/Rx pair. Using SSF propagation simulations and related calculations, the validation process may take minutes per OCh trail, and many hours for a network. Often, this is unacceptably long for OPT users, and also too long for automated use in path-validation algorithms within optical control planes (OCPs).
Techniques that enable rapid validation of optical channel trails remain highly desirable.